Study Of Topological Properties In Transition-Metal Dipnictides

Since the integer quantum hall effect in two-dimension(2D)electron gas was discovered in the 1980s,topological properties of solid state materials have became a new research spot 2D and 3D topological insulators(TIs),Dirac/Weyl semimetals,topological superconductors were predicted theoretically and verified experimentally in succession.Among these new compounds,dipnictides attract extensive attention due to their extremely large magnetoresistance,and non-trivial properties in these dipnictides according to theoretical calculations.This thesis consists of seven chapters,and the first chapter introduces the concept of different topological materials and basic transport theories.The second chapter introduces the methods of synthesizing samples,structural characterization,and transport property measurements.The third chapter introduces several topological non-trivial semimetals of transition-metal dipnictides MPn2(M = Nb,Ta;Pn = As,Sb),and studies different tansport features in different electron-hole(e-h)compensation systems.The fourth chapter studies the pressure-induced superconductivity in topological semimetal NbAs2,which is a good example to study the transition between different topological phases and provides a candidate for topological superconductors.The fifth chapter introduces quantum transport properties in a new topological semimetal W2As3.The sixth chapter introduces other works during Ph.D period.The seventh chapter introduces summary and outlook about these topological materials of transition-metal dipnictides.In addition,the main innovative results are listed as the followings:(1)Topological semimetals MPn2(NbAs2,TaAs2 and TaSb2)host different electron-hole(e-h)compensation effect,thus have different transport properties.According to the first-principles calculations,the topological non-trivial Z2 indices is[0;111],indicating a weak TI feature.Ex-tremely large magnetoresistance(MR)is observed in these three compounds,and MR in NbAs2,TaAs2 and TaSb2 is nearly 2100,2000 and 17000 at 1.5 K and 15 T,respectively.From the fitting for MR,we observe more perfect e-h compensation in TaSb2 than that in NbAs2 and TaAs2.This is also testified by Hall resistance.The Hall resistance of NbAs2 and TaAs2 is nonlinear at the whole range of magnetic field(B),and the absolute value of Hall resistance is very large at 15 T.According to the two-band model,Hall resistance is linear with magnetic field if perfect e-h com-pensation existed.Although the nonlinear Hall resistance in TaSb2 appears at high magnetic field and low temperature,Hall resistance is linear at low magnetic field and the absolute value is very small.Moreover,Nernst signal of TaSb2 shows more perfect linear behavior compared with NbAs2 and TaAs2.So we conclude that TaSb2 is the most perfect e-h compensation system in these three samples.When magnetic field is parallel with electrical field(E),the upturn of resistance at low magnetic field and the negative magnetoresistance at high magnetic field are detected.We suggest the upturned resistance is caused by weak anti-localization.According to theoretic calculations,magnetic field could induce the Weyl points in MPn2,which may be helpful to explain the negative magnetoresistance.(2)We report pressure-induced superconductivity(SC)in a transition-metal dipnictide NbAs2.The emergence of SC is not accompanied by any structural phase transition up to the maximum ex-perimental pressure of 29.8 GPa,as supported by pressure-dependent synchrotron X-ray diffraction and Raman spectroscopy.Intriguingly,the Raman study reveals rapid phonon mode hardening and broadening above 10 GPa,in coincident with the SC transition.Using first-principle calculations,we determine Fermi surface change induced by pressure,which steadily increases the density of states without breaking the e-h compensation.Noticeably,the main hole pocket of NbAs2 enclos-es one time-reversal-invariant(TRI)momentum of the monoclinic lattice,suggesting NbAs2 as a candidate of TRI topological superconductors.(3)We report W2As3,a topological semimetal with non-trivial properties according to theo-retical calculations,whose Z2 indices is[1;111],suggesting a strong TI feature.Interestingly,the MR in W2As3 can reach 11500%at 3 K and 15 T with good e-h compensation.Apparent quan-tum oscillation can be detected under high pulse magnetic field up to 60 T.From the fast Fourier transform(FFT),we get nine intrinsic frequencies when B is applied along(200)direction.A non-trivial π Berry’s phase is obtained in Fh3,Fh4 and Fh5.Many frequencies in FFT spectroscopy,Hall resistance and band calculations illustrate the multiband features.Therefore,we use a simple two-band model to fit the conductivity and Hall coefficient and the fitting is well.The calculat-ed Hall coefficient is consistent with experimental data.All of these show a reliable fitting by a two-band model in W2As3.At last,we also briefly introduce two other works during the Ph.D period,including:(1)Neg-ative magnetoresistance in Weyl semimetals NbAs and NbP:Intrinsic chiral anomaly and extrinsic Effects;(2)Magnetoresistance of multi-pocket nodal-line fermions in HfSiS.